Generally the invention relates to the field of solid state magnetic resonance spectroscopy. More specifically, the invention relates to the narrow field of variable-temperature magnetic resonance spectroscopy of solids.
Magnetic resonance spectroscopy is an aspect of analytical chemistry which includes Nuclear Magnetic Resonance (NMR) spectroscopy. This technique is used to determine the characteristics of a particular material and to identify basic structures and compositions of that material. It is based upon the well-known fact that when substances are subjected to electromagnetic radiation of the correct frequency and orientation, they may respond by emitting radiation which is characteristic of the particular molecular and atomic structure of that substance. In order to sense this effect for substances in a solid state, the material must be rotated at high frequency in an intense magnetic field In addition, the temperature of the material may need to be accurately set.
Historically the technique of magnetic resonance spectroscopy was first developed in 1948 for materials in the liquid state. This development eventually lead to the Nobel Prize. As part of the technique for analyzing materials in the liquid state, the materials were placed in an intense magnetic field and at times rotated at frequencies of up to 100 hertz (6,000 rpm). While rotation is not fundamentally necessary to analyze liquids, it was developed as a technique to overcome experimental problems caused by gradients in the external magnetic field. Although analysis of substances in the solid state was attempted initially using similar techniques, solids simply do not react to that external stimuli as liquids do. Since NMR spectroscopy of materials in the liquid state was relatively simple and readily yielded valuable information, the field of liquids NMR moved rapidly away from the theory-based scientists who originated it to application-oriented persons. The techniques of liquid NMR simply became sufficiently understood that theorists were minimally involved.
Theorists did, however, remain interested in the difficulties encountered in the magnetic resonance of materials in the solid state and, in the mid 1960s fundamental breakthroughs occurred. These breakthroughs were based upon the realization that an important difference was necessary to achieve results with sufficient resolution. Simply put, it became generally understood that it was not enough to merely rotate the sampling in a magnetic field. Rather, the sample had to be oriented with respect to the magnetic field (the so called "magic angle") and that the sample had to be rotated at extremely high frequencies--on the order of many kilohertz (hundreds of thousands of rpm). Rather than only representing a change in degree with respect to the way liquids NMR was done, the very fast rotation of a solid sample was not necessitated by experimental difficulties as in liquids NMR. Instead, it was due to fundamental reasons which resulted in reducing the nuclear spin interactions of the material. This fundamental difference coupled with the application-oriented development of liquids NMR resulted in a separation between those persons skilled in the art in liquid state NMR and those skilled in the art of solid state magnetic resonance. Although related by the fact that both were based upon the general principle of magnetic resonance, those skilled in the two arts had little overlap; indeed, their technical backgrounds were unusually quite diverse.
As the technique of solid state magnetic resonance was refined, many practical difficulties were encountered. Many of these were of such a nature that they prevented broad scale commercial acceptance of solid state magnetic resonance for some time. While certain facets of solid state magnetic resonance became accepted, other facets posed peculiar difficulties. One of these areas was the field of variable-temperature solid state magnetic resonance. This field has only recently developed. One of the obstacles faced in its development is that of accurately and uniformly controlling the temperature of the solid material to be analyzed. Since virtually all samples in solid state magnetic resonance are spun through use of gas-driven rotors, it was only natural that those skilled in the art of solid state magnetic resonance focused on the gas drive to control sample temperature. While it seems simple enough to merely change the temperature of the spin gas or gases (frequently a bearing gas and a drive gas were separately supplied), this resulted in other problems.
Perhaps the primary difficulty in the approach of controlling temperature with the spin gases was the fact that changes in temperature also resulted in changes in the frequency of rotation. Not only was this due to variation in the density of the spin gas, but it was also due to the fact that the combination of pressures and temperatures involved approached that conducive to condensation of the spin gas at the colder temperatures and unacceptably decreased gas density at higher temperatures. In addition to these effects, the problems of impacting the temperatures of the electronics in the vicinity of the rotor and the magnet (typically held at superconducting temperatures) made variable-temperature solid state magnetic resonance spectroscopy a unique problem. As an example, prior to the present invention, temperature/rotational frequency values ranged from 20.degree. C./8 Khz to -50.degree. C./6 Khz to -150.degree. C./2 Khz in spite of efforts by those skilled in the art to avoid any change in frequency over such a temperature range.
An additional problem encountered by those developing the field of variable-temperature solid state magnetic resonance spectroscopy was the problem of accurate and uniform temperature control of the sample. Since temperature sensing was primarily achieved by sensing the temperature of the gas supplied to the rotor, it was not a direct representation of the temperature of the sample. This aspect and others were explained in the article "Experimental Considerations in Variable-Temperature Solid State Nuclear Magnetic Resonance With Cross Polarization and MagicAngle Spinning" by James F. Haw, et al., 58 Analytical Chem. 3172 (1986). The many different effects present resulted in not only inaccurate sample temperature estimation but also in nonuniform sample temperature. To avoid this, those skilled in the art have been seeking to minimize and avoid any temperature gradient within the rotor, unlike the techniques of the present invention. The temperature non-uniformity also had a direct effect on signal resolution in variable-temperature work. While efforts were made to independently control the bearing gas temperature and the drive gas temperature to overcome some effects, this approach generally resulted in unacceptable nonuniformity of the sample temperature and thus signal resolution decay. Hence, those skilled in the art tried to avoid temperature gradients.
In order to solve these and other problems encountered in variable-temperature solid state magnetic resonance spectroscopy, the present invention was developed. By supplying a separate sample gas in a particular fashion, it was discovered not only that accurate temperature control and uniformity could be achieved within the sample itself, but also that the undesirable effect of sample temperature on rotational frequency could be virtually eliminated. In addition, the present invention provides a design which substantially enhances the energy and cost concerns of performing solid state magnetic resonance spectroscopy. In this regard, it should be understood that analysis occasionally entails running a sample for many days at an extremely cold temperature. Obviously the costs using conventional techniques could become a concern. In addition, in the event relatively expensive cryogenic gases are utilized to overcome the condensation limits of nitrogen, operational expenses for such analysis can become an important factor.
While the basis of the present invention could be considered to be relatively simple, it is a fact that those skilled in the art of solid state magnetic resonance failed to realize the proper combination and selection of elements to overcome the prior limitations. Although the implementing arts and elements of the present invention were available, those in the field focusing on the problem of proper sample temperature control had not been able to solve the problem. The preconception that spin gas temperatures were necessarily tied to sample temperature control resulted in those skilled in the art teaching away from the direction of the present invention. Those skilled in the art simply failed to realize that the problem lay in assuming that sample temperature was tied to spin gas temperature. While there had been substantial attempts by those skilled in the art at overcoming the problems of variable-temperature solid state magnetic resonance spectroscopy, until the present invention such attempts had not resulted in a complete solution to the problem. This seems especially highlighted by the fact that the present invention has been very well received by researchers in the field even though only nominal effort and funds have been spent to market it. These same researchers have also expressed disbelief that such a simple system works so effectively, was so readily at hand, and yet went unrealized by those skilled in the art.